This invention relates to a cathode ray tube and, more particularly, to a cathode ray tube having a reflection prevention film formed on a face plate.
The cathode ray tube usually has a smooth glass surface as its outer surface. Therefore, ambient light rays incident on the outer surface acting as a mirror surface are reflected so that the image on the face plate can not be seen clearly.
There are two well-known methods for solving such a problem. In one of these methods, the outer surface of the face plate is formed with fine irregularities so that ambient light rays are scattered by these irregularities, as disclosed in Japanese Patent Laid-Open No. 61-29051. In this method, light rays are randomly reflected by the entire screen. Therefore, the screen as a whole is seen rather whitish, and the contrast seems to be deteriorated. Further, the resolution of the image is liable to be deteriorated. In the second method, a reflection prevention film having a single-layer or multiple-layer structure is formed on the outer surface of the face plate for preventing the reflection, as disclosed in Japanese Patent Laid-Open No. 61-91838. The reflection prevention film is usually made of a material having a refractive index lower than the refractive index of the glass material of the face material. The optimum thickness of the film is .lambda./4n where .lambda. is the wavelength of light rays, the reflection of which is to be prevented, and n is the refractive index of the film. For example, where a film of magnesium fluoride is formed on the face plate to prevent the reflection of light rays with a wavelength of 0.55 .mu.m, the thickness of this film is set to 0.1 .mu.m for the refractive index of magnesium fluoride is substantially 1.38. With the face plate with such a reflection prevention film, the reflection prevention effect is different with the central and peripheral areas. That is, light rays incident on the face plate are seen in different colors in the central and peripheral areas of the face plate, for instance, in purple in the central area and in blue in the peripheral areas of the face plate. This phenomenon is undesired in a cathode ray tube from the standpoint of the color reproducibility.